Assessment of sources variability of riverine particulate organic matter with land use and rainfall changes using a three-indicator (δ¹³C, δ¹⁵N, and C/N) Bayesian mixing model

In intensive agricultural watersheds, riverine particulate organic matter (POM) may be transported from many sources such as rice paddies, crop uplands, forests, and livestock farming areas during rainy seasons. However, the impacts of land-use and rainfall changes on the POM sources are not well understood. In this study, changes in the sources of riverine POM were investigated in an agricultural area of Korea between 2014 and 2020/21. During this period, land-use and rainfall patterns changed dramatically. The δ¹³C, δ¹⁵N, and C/N of the POM sources as well as those of riverine POM were analyzed, and a stable isotope analysis in R (SIAR) model was utilized for source apportionment. There were differences in δ¹³C, δ¹⁵N, and C/N among the sources. For example, manure had higher δ¹³C (−22.6 ± 3.3‰) and δ¹⁵N (+10.6 ± 5.9‰) than soils (from −28.0 ± 0.8‰ to −25.1 ± 1.2‰ for δ¹³C and +3.6 ± 1.7‰ to +9.8 ± 1.4‰ for δ¹⁵N). For soils, the δ¹³C and δ¹⁵N were higher for upland soils, while C/N was greater for forest soils than for others. For riverine POM, the δ¹⁵N marginally changed; however, the δ¹³C and C/N increased from −26.1 ± 0.9‰ to −20.8 ± 5.3‰ and from +7.7 ± 1.7 to +18.8 ± 8.3 between 2014 and 2020/21, respectively. The SIAR model showed that the contributions of paddy (from 41.0% to 14.9%) and upland fields (from 48.1% to 23.7%) to riverine POM decreased between the periods due to decreased paddy area and the implementation of best management practice on upland fields, respectively. However, the contribution of forests (from 3.5% to 28.0%) and manure (from 7.4% to 33.5%) increased probably due to improper management of forest clear-cutting sites and livestock manure storage sites. The contributions of agricultural soils to riverine POM decreased in drier years. Our study suggests that land management rather than land-use area is critical in riverine POM management, particularly in wetter years.